With the widespread deployment of fiber optic transmission systems and the alarming rate of outages due to fiber cuts, there is great interest in improving the process of restoring disrupted traffic from minutes to sub-seconds following a fiber or span cut. Automatic protection switching probably is the fastest technique and can switch the disrupted traffic to dedicated spare links in under 50 milliseconds. However, it also requires high dedicated spare channel capacity. With recent advances in digital cross connect systems (DCS), there is increasing interest in using DCS in network restoration.
There are two basic approaches to reroute the disrupted traffic due to a fiber span cut. The link restoration approach replaces the affected link segment of a disrupted channel by a spare path between the two disrupted ends. The path restoration approach releases each disrupted channel and lets the source and destination end of the channel re-establish the connection. With the additional release phase the path restoration approach takes more time than the link restoration. However, the path restoration approach can find more efficient spare paths with fewer link segments and can handle the node failure situation with the same logic. The link restoration approach has applications in achieving fast network restoration.
A well-known distributed network restoration method for a DCS-based fiber network was proposed by W. D.
Grover in "The Self-Healing Network: A Fast Distributed Restoration Technique For Networks Using Digital Cross-Connect Machines", Proc. GLOBECOM '87, pp. 28.2.1-28.2.6, 1987 and detailed in his 1989 Ph.D. dissertation for the Department of Electrical Engineering at University of Alberta entitled, "Self Healing Networks: A Distributed Algorithm For K-Shortest Link-Disjoint Paths In A Multi-Graph With Applications In Real Time Network Restoration." The protocol associated with that process is called the Self-Healing Network (SHN) protocol.
Another distributed network restoration process for DCS-based fiber networks has been proposed by Yang and Hasegawa in "FITNESS: Failure Immunization Technology for Network Survivability," Proc. of GLOBECOM '88, pp. 47.3.1-47.3.6, November 1988. This method became known as Bellcore's FITNESS approach
RREACT is another distributed approach to network restoration and is described in detail in "RREACT: A Distributed Protocol for Rapid Restoration of Active Communication Trunks", UCCS Tech Report EAS-CS-92-18, November 1992.
The above approaches are not sufficient as they appear to be unstable and result in an explosion of messages throughout the network following the failure of a network link or node. These approaches also rely upon extensive knowledge of the network topology prior to the failure. Unfortunately, the information could be out of date at the time of the failure, especially in the situation of a multiple link failure. The complexity, instability, and unreliability make these approaches unsatisfactory for many types of distributed telecommunications networks.